Field of the Invention
The present invention relates to a motor control apparatus which controls drive of a motor, and particularly to a motor control apparatus which controls drive of a brushless motor including an encoder which detects rotation of a rotor of the brushless motor.
Description of the Related Art
Performing feedback control of a brushless motor provided with an encoder which detects rotation of a rotor thereof by using an output signal from the encoder enables driving the motor at a high velocity without causing loss of synchronization thereof. Such a brushless motor which can be driven at a high velocity requires in its control a stop position accuracy to stop a driven member to be driven by the motor accurately at a target stop position.
For ensuring such a stop position accuracy, an acceleration (deceleration) of the motor in its deceleration state from its constant-velocity state or its acceleration state and a rotation amount of the rotor in the deceleration state, namely, a drive amount of the driven member (hereinafter referred to as “a deceleration drive amount”) are important. A higher deceleration in the deceleration state enables decelerating the driven member toward the target stop position in a shorter time. This means that such a higher deceleration enables reducing the deceleration drive amount, that is, increasing a drive amount of the driving member in the constant velocity state or the acceleration state, which makes it possible to reduce a drive time required for driving the driven member to the target stop position. However, a high deceleration with a small deceleration drive amount may result in a situation that the driven member stops beyond the target stop position.
Japanese Patent Laid-Open No. 2004-129452 discloses a technique which switches an energization phase of a motor with increase of a count value of pulse signals output from an encoder in response to rotation of the motor to control drive of the motor. More specifically, this technique corrects a phase advance amount, which is a phase difference of the energization phase from a rotational phase of a rotor of the motor, to adjust a braking force, namely, an acceleration (deceleration), which results in an improvement in stop position accuracy. Similarly, Japanese Patent Laid-Open No. 8-63228 discloses a technique which changes a timing of outputting a stop instruction for a movable member based on a velocity variation time in which the movable member is accelerated or decelerated from a constant velocity state toward another velocity state or on a velocity variation time in which the movable member is accelerated from a stop state to a predetermined velocity state. This technique aims to stably ensure a stop position accuracy even in case of occurring a sudden environmental change or an aging variation.
The technique disclosed in Japanese Patent Laid-Open No. 2004-129452 corrects the phase advance amount, taking a rotational angle of the motor from a current position to a target position into consideration. That is, if a timing of decelerating the rotor delays due to any causes, the technique increases the braking force applied to the rotor to perform deceleration control before the target stop position. However, since the acceleration depends on the braking force of a driving system, the upper limit thereof the acceleration is limited depending on a system concerned. Moreover, the upper limit varies due to manufacturing errors. The technique disclosed in Japanese Patent Laid-Open No. 2004-129452 does not take into consideration not only a difference of the braking force caused by an individual difference and an operation state of the driving system, but also the deceleration drive amount.
On the other hand, the technique disclosed in Japanese Patent Laid-Open No. 8-63228 sets a deceleration and stop timing based on the velocity variation time in which the movable member is accelerated or decelerated for its velocity change or is accelerated from the stop state and on a velocity variation and a velocity deviation while a constant-velocity-operation instruction is given. That is, this technique is designed to ensure the stop position accuracy by obtaining characteristics changes due to the aging variation or the like and by appropriately setting the deceleration drive amount. However, this technique is not enough to ensure the stop position accuracy in a case where a constant velocity state is not present because this technique requires the presence of the constant velocity state during the drive of the movable member as a precondition.